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A construction scheme for heralding virtual graph states, with emit-then-add. A quantum emitter (red) is repeatedly excited until a single photon (orange) is successfully detected. Upon detection, logic is executed which passes the conditional phase information written on to the emitter to an auxiliary memory spin (black), which encodes a virtual graph state (represent by dashed boundaries). Credit: Maxwell Gold et al
Physicists have long recognized the value of photonic graph states in quantum information processing. However, the difficulty of making these graph states has left this value largely untapped. In a step forward for the field, researchers from The Grainger College of Engineering at the University of Illinois Urbana-Champaign have proposed a new scheme they term "emit-then-add" for producing highly entangled states of many photons that can work with current hardware. Published in npj Quantum Information, their strategy lays the groundwork for a wide range of quantum enhanced operations including measurement-based quantum computing.
Entanglement is a key driver in delivering faster and more secure computational and information systems. But creating large, entangled states of more than two photons is challenging because the losses inherent in optical systems mean most photon sources have a low probability of successfully producing a photon that survives to the point of detection. Therefore, any attempt to build a large entangled state is full of missing photons, breaking the state apart. And identifying the missing spots would mean attempting detection of the photons, which is a destructive process itself, and precludes going back to fill those spots.
To circumvent this challenge, a team led by Associate Professor of Physics Elizabeth Goldschmidt and Professor of Electrical and Computer Engineering Eric Chitambar began with a different mindset.
Rethinking what is 'good enough'
"We've known for years that these photonic graph states are incredibly valuable," Goldschmidt said. "But for this project, we changed our thinking from, "What would be the most useful end result?" to, "What can we do with the resources we already have?" It took us a long time to realize that destructively measuring the photons would be okay for this wide set of useful circumstances."
This reframing led the researchers to posit a heralded scheme for making photonic graph states—one compatible with state-of-the-art coherent quantum emitters. The key? A paradigm they term "virtual graph states." By adding a photon to a virtual graph only after its confirmed detection, the process's primary limitation shifts from photon loss—which is generally large—to the coherence of the spin qubits used to emit the photons, which can be very long.
Virtual graph states in practice
The Illinois Grainger engineers highlight that their protocol is fully general if non-destructive measurement of the photons can be implemented—however, this is out of reach with current hardware. Thus, they introduce a broad class of protocols that can be implemented successfully using destructively measured photons and virtual graph states. They propose an example-use case that could be implemented on currently available standard experimental apparatuses to perform secure two-party computation based on repeatedly generating small graph states.
"There's something almost counterintuitive about it," said Max Gold, a graduate student and co-lead author of the paper. "We're building up these correlations that can only be described by quantum systems across different photons. We have these photons that don't ever exist at the same time in nature, and something mediating their interactions that's not the photons themselves. Even though we talk about it as a single state, not all the qubits in the state exist at one time."
Potential applications and next steps
If implemented, the researchers' methodology could be used in measurement-based quantum computing, secure two-party computation, and even quantum sensing.
"This could be done on a number of experimental apparatuses around the world," said Jianlong Lin, a graduate student and co-lead author of the paper. "Our method is feasible in practice even for emitter-based platforms with traditionally low photon collection efficiencies such as trapped ions and neutral atoms. It would be one of very few demonstrations of photonic graph states with practical uses."
Going forward, Goldschmidt's lab is working to realize their protocol. Lin will continue focusing on the experimental side, initiating the early stages of the process. Meanwhile, Gold will explore the theoretical side by identifying other potential applications for graph states using their methodology.
"We've created a protocol based on (realistic) hardware that has at least one use, which is this multi-party computation," Goldschmidt said. "A lot of the literature has ignored hardware limitations, and I hope this work encourages other people to think about what could be produced given the real constraints of real near-term hardware."
— Source: Phys.org (https://phys.org/news/2026-02-physicists-protocol-photonic-graph-states.html)
